A non-volatile memory such as MRAM using a magnetoresistance effect element is known (for example, see PTL 1). A magnetoresistance effect element can be exemplified by a magnetic tunnel junction element (MTJ element) in which a barrier layer 112 (nonmagnetic barrier layer) is disposed between a reference layer 111 and a recording layer 113 which are magnetic layers as shown in FIG. 16. In the example shown in FIG. 16, the reference layer 111, in which the direction of magnetization hardly changes, is magnetized in the direction perpendicular to the layer surface (film surface). The magnetization direction of the recording layer 113 is variable. The saturation magnetization Ms of the recording layer 113 is defined by the material, structure, temperature, and the like of the recording layer 113. The magnetoresistance effect element shown in FIG. 16 is a so-called “perpendicular anisotropy MTJ element”. When the magnetization direction of the recording layer 113 is parallel to the magnetization of the reference layer 111, the electrical resistance of the MTJ element decreases, and when the magnetization direction of the recording layer is antiparallel to the magnetization of the reference layer, the electrical resistance of the MTJ element increases. The MTJ element has a structure in which information can be recorded by associating the two states of resistance with “0” and “1” of bit information, respectively.
As shown in FIGS. 16 and 17(a), an energy barrier Eb of the recording layer 113 can be expressed by an Equation (1) by using an angle θ formed by the magnetization direction of the recording layer 113 and the magnetization direction of the reference layer 111, a magnetic anisotropic energy density Keff of the reference layer 111, and a volume V of the recording layer 113. When sin2 θ=1 (θ=90°,270°), this energy becomes the energy barrier (Eb) required for magnetization reversal.Eb=KeffV sin2 θ  (1)
For a non-volatile memory such as a MRAM having a magnetoresistance effect element (MTJ element), a thermal stability index is an indicator of stability of bit information. This thermal stability index Δ0 is expressed by an Equation (2) using a Boltzmann constant kB and an absolute temperature T.Δ0=Eb(kBT)  (2)
The probability P that a recording layer having a thermal stability index Δ0 will demonstrate magnetization reversal after a time t is expressed by an Equation (3) according to a Neel-Arrhenius law (for example, see NPL 1). As shown in FIG. 17(b), the time t when the probability P is 50% corresponds to the retention time of information by the recording layer 113.P=1−exp{(−t/10−9)×exp(−Δ0)}  (3)
A magnetic field pulse method, a current pulse method, and the like are known as general methods for measuring the thermal stability index Δ0. In the magnetic field pulse method, the magnetization reversal probability when a magnetic field pulse of a specific pulse width is applied is measured while changing the magnitude of the magnetic field of the magnetic field pulse, and the thermal stability index Δ0 is obtained based on the relationship between the magnitude of the magnetic field and the magnetization reversal probability.
In the current pulse method, the magnetization reversal probability when a current pulse of a specific pulse width is applied is measured while changing the magnitude of the current of the current pulse, and the thermal stability index Δ0 is obtained based on the relationship between the magnitude of the current and the magnetization reversal probability.
A measurement means for measuring the thermal stability index Δ0, which is different from the magnetic field pulse method and current pulse method, is known in which an MRAM chip including MTJs of 10 Mb or more is prepared, 1 or 0 information is written in a checkerboard pattern or the like in the recording layer of each MTJ, the chip is allowed to stand at high temperature for several minutes to 100 hours, an error rate indicating how much initial information has been lost is measured, and the thermal stability index Δ0 is obtained based on the relationship between the reversal probability calculated from the error rate and the standing time. According to this method, since the value of the energy barrier does not change during measurement, an accurate thermal stability index Δ0 can be obtained regardless of the mode of magnetization reversal.
For example, a method for producing a magnetic memory using a magnetoresistance effect element is known, in which a complementary mental oxide semiconductor (CMOS) is formed in a wafer serving as a substrate, an intermediate wiring for connecting to an MTJ on the CMOS is formed, a magnetic film is wired to the upper portion of the intermediate wiring, the magnetic film is subjected to heat treatment in a magnetic field, an MTJ pattern is thereafter prepared, an MTJ is formed by etching process, a protective film is formed, an upper wiring is formed on the MTJ, the configuration is cut into a chip shape and connected to a predetermined circuit substrate by a wire bonding metal wire, and then resin sealing is performed.